Hyperplastic atypical lobules (AL) in human breasts are hypothesized to be precancerous on the basis of circumstantial evidence. The purpose of our work is to test the hypothesis by transplanting presumed precancerous AL, normal lobules and ducts, and cancer into the host gland free fat pads of "nude" athymic mice. The transplanted tissues are left in the host for variable times, the animals sacrificed, and the transplants examined for evidence of growth. Various hormonal manipulations are used to enhance growth.